This invention is concerned with fuel cells utilizing a liquid electrolyte and having an electrolyte containing matrix layer disposed between a gas diffusion anode electrode and a gas diffusion cathode electrode. These fuel cells can be oriented adjacent one another and electrically connected, typically in series, to form a fuel cell stack. During operation of the cells, where, for example, hydrogen and oxygen are reacted to produce electrical energy, water and heat, cooling with liquid or gaseous cooling fluids is required to maintain component integrity. An example of one type of fuel cell system configuration is taught by R. Kothmann et al., in U.S. Pat. No. 4,276,355, where the electrodes are made from a porous graphite material with a porous graphite fiber backing, and a portion of the matrix is in the form of porous graphite saturated with phosphoric acid electrolyte. Operation of such modern phosphoric acid fuel cells is usually within a temperature range of from about 175.degree. C. to about 225.degree. C.
In very early work in the fuel cell area, inorganic ion transport membranes were utilized to allow fuel cell operating temperatures of from about 75.degree. C. to about 100.degree. C. These membranes were about 0.03 inch thick and consisted of ZrO(H.sub.2 PO.sub.4).sub.2, zirconium phosphate, i.e., zirconium orthophosphate, having active ion exchange properties, compressed under high pressure with polytetrafluoroethylene, i.e., Teflon. Each side of this inorganic ion exchange membrane was pressed to a platinum black catalyst layer and a covering platinum screen electrode. Operation was characterized as: hydrogen fuel donating an electron as the hydrogen ionized at an anode electrode screen, catalyzed by the platinum black; then, hydrated by one or more molecules of water, the hydrogen ion diffused into the ion exchange membrane and was transferred across available proton sites on the PO.sub.4 groups. Such work was reported in Chemical & Engineering News (C&EN), Oct. 16, 1961 issue, at page 40.
A major problem with the ZrO(H.sub.2 PO.sub.4).sub.2 ion exchange membrane was poor transverse strength, and its low maximum operational temperature of about 100.degree. C. These problems were solved in the mid 1960's by C. Berger et al., in U.S. Pat. No. 3,462,314. C. Berger et al. taught making the inorganic ion exchange membranes by sintering a mixture containing: water balancing agent, such as aluminosilicate, aluminum sulfate, silica gel, copper sulfate, calcium chloride, and the like; water insoluble hydrous metal oxides or water insoluble acid salts, such as hydrous zirconium dioxide; and inorganic acid, such as phosphoric acid, boric acid, molybdic acid, sulfuric acid, and the like, at a temperature of from about 200.degree. C. to 1000.degree. C., preferably from about 300.degree. C. to 600.degree. C., at pressures of up to about 10,000 psi. This compacted, sintered, inorganic ion exchange membrane was then saturated with inorganic acid and resintered within the previously used temperature range, to provide strong bonding of the water balancing agent. The resintering operation provided high transverse strength and provided a membrane capable of operational temperatures up to 125.degree. C. The examples show equal parts of ingredients, sintering at 400.degree. C. for 5 hours and resintering at 500.degree. C. for 2 hours, to produce an inorganic ion exchange membrane consisting essentially, of, for example, ZrO(H.sub.2 PO.sub.4).sub.2 with bonded aluminosilicate. If any ZrP.sub.2 O.sub.7 were produced, it was incidental or unnoticed, and not mentioned. This ion exchange membrane was disposed between dual, platinum black catalyst-tantalum electrode screen layers.
At about the same time as the work of C. Berger et al., O. Adlhard et al., in U.S. Pat. No. 3,453,149, taught porous matrix, acid electrolyte containing, flexible, 0.02 inch to 0.025 inch thick membranes for fuel cells, capable of operational temperatures of about 165.degree. C. The matrix component was made from oxides, sulfates or phosphates of zirconium, tantalum, tungsten, chromium or niobium, mixed with an aqueous polytetrafluorethylene dispersion, and phosphoric acid. These materials were homogeneously mixed, for example, 4 moles of 100% phosphoric acid to 1 mole of zirconium oxide, plus a 40% aqueous emulsion of polytetrafluoroethylene; heated at 200.degree. C. to coagulate the polytetrafluorethylene emulsion, which acts as a binder; and then pressed into sheet form. Platinum black powder could then be pressed at 1000 psi onto both sides of the matrix sheet, and platinum mesh screens attached on both sides to the platinum powder. At such a high molar excess of phosphoric acid, diluted with the aqueous polytetrafluoroethylene carrier, and heated at only 200.degree. C., the matrix would presumably consist in major part of ZrO(H.sub.2 PO.sub.4).sub.2, and unreacted ZrO, with unbound, condensed pyrophosphoric, tripolyphosphoric and tetrapolyphosphoric acids. If any ZrP.sub.2 O.sub.7 were produced, it was incidental or unnoticed, and not mentioned. Such matrix membranes were relatively thick, had a cement consistency subject to cracking even with polytetrafluoroethylene inclusion, were reactive with phosphoric acid electrolyte, and still operated at a limiting, low temperature.
At a somewhat later date, as part of a NASA project dealing with sintered zeolite membranes for fuel cells, A. Levy-Pascal, C. Berger, A. Kelmers, N. Michael, and M. Plizga investigated X-ray diffraction analyses of ion exchange membranes made from a mixture of equal molar portions of ZrO.sub.2, an acid stable form of zeolite (Na.sub.2 O.Al.sub.2 O.sub.3.nSiO.sub.2.xH.sub.2 O) and H.sub.3 PO.sub.4. The membranes were fired at different temperatures, and x-ray diffraction analyses were then made on the fired membranes. Membranes fired at from 300.degree. C. to 700.degree. C. contained ZrO.sub.2, zeolite, plus unidentified phases. One membrane of the same equal molar portions of ZrO.sub.2, zeolite and H.sub.3 PO.sub.4 fired at 600.degree. C. showed phases of ZrO.sub.2, ZrP.sub.2 O.sub.7 (zirconium pyrophosphate), and zeolite, while one other fired at 1040.degree. C. showed phases of ZrO.sub.2 and ZrP.sub.2 O.sub.7 (Tables XI and XII) with all the zeolite apparently destroyed. Any zirconium pyrophosphate produced was incidental and noted in passing, but no advantageous properties were recognized. All of the fired membranes contained major amounts of unreacted ZrO.sub.2. This work was reported by Levy-Pascal et al., "Investigation of Zeolite Membrane Electrolytes For Fuel Cells" Report 108-Q3, NASA, March 1963. In the membrane fired at 1040.degree. C., with equal molar ratios of starting materials used, ZrO.sub.2 seemingly would constitute from 50 wt.% to 99 wt.% of the membrane, with ZrP.sub.2 O.sub.7 present at less than 50 wt.%, since 1 mole of ZrO.sub.2 +1 mole of H.sub.3 PO.sub.4 could only produce 1/2 mole of ZrP.sub.2 O.sub.7 even assuming all of the P reacts with the Zr.
Additional membrane research in the area of molecular sieve type materials. A. Clearfield in U.S. Pat. No. 4,059,679, taught preparation of crystalline materials from phosphate gels. The reflux product of zirconyl chloride and orthophosphoric acid, when washed and then dried below 100.degree. C. was found to be a highly crystalline compound corresponding to Zr(HPO.sub.4).sub.2.H.sub.2 O.
R. Breault, in U.S. Pat. No. 4,017,664, after reviewing the necessary properties of a phosphoric acid fuel cell matrix, i.e., it must be porous, thin, liquid permeable, wettable, have good ionic conductivity, be electrically insulative, and be chemically stable at high temperatures and open circuit potentials with a bubble pressure sufficient to prevent reactant gas crossover; rejected all prior matrix materials in favor of silicon carbide. In Breault, a 10:1 weight ratio of inert silicon carbide powder under 25 micron particle size:polytetrafluoroethylene dispersion, was applied as a layer to the surface of the catalyst side of a platinum black-polytetrafluoroethylene electrode. The layer was dried to remove water, pressed at 200 psi and sintered at 310.degree. C. for 5 minutes, to provide a 0.04 inch thick silicon carbide-polytetrafluoroethylene matrix, essentially unreactive with phosphoric acid electrolyte and capable of operational temperatures of from about 175.degree. C. to 200.degree. C. While a silicon carbide matrix is well accepted, there is still need for improvement in terms of a low cost, inactive, thinner matrix material, providing high performance at higher temperatures.